This invention relates to a demodulator and more particularly to a baseband demodulator for FM signals.
In regard to the field of radio receivers there has been a concentrated effort over the past several years to reduce the amount of tuned circuitry employed in such devices. In obtaining a reduction in the number of tuned circuits one is able to integrate large portions of the receiver and hence produce radio receivers which are extremely compact. These receivers are employed in many areas such as in selective paging systems and so on. A major breakthrough in the design of such receivers is a technique referred to as "zero -IF."
The zero-IF technique has been described in British Patent No. 1,530,602 entitled DEMODULATOR FOR FM SIGNALS published on Nov. 1, 1978 for I. A. W. Vance. Essentially in such a system, there is present a receiver in which local oscillator signals in phase quadrature at the carrier frequency are each separately mixed with the incoming audio modulated signals. The resulting signals have zero-IF with the two sidebands folded over on each other at the baseband and extending in frequency from DC to the single sideband width of the original signal.
In the case of FM modulation the folding of the sidebands makes it impractical to demodulate the signal. In the British Patent the mixer outputs are low pass filtered and then amplified to a standard limit. After amplification, the two signals are separately differentiated. Each differentiated signal is then multiplied by the input to the other differentiator and one of the multiplier outputs is substracted from the other. This technique is also described in U.S. Pat. No. 4,238,850 entitled TRANSMITTER/RECEIVER FOR SINGLE CHANNEL DUPLEX COMMUNICATION SYSTEMS issued on Dec. 9, 1980 to I. A. W. Vance and assigned to International Standard Electric Corporation.
In such systems, one may employ a phase-locked loop demodulator operating at the carrier frequency. The local oscillator of such a system follows the signal and the control voltage for such a local oscillator is therefore proportional to the signal deviation plus a constant. If the constant is removed, as for example by coupling, then the output is a replica of the deviation and therefore represents the modulation.
The difficulty with the demodulators of the prior art is that the stability of the criteria for the phase-locked loop prohibits sharp filters in the baseband. Accordingly, the adjacent channel selectivity suffers degradation and therefore such a receiver is extremely difficult to implement. Hence in recognizing this problem many prior art systems use two channels in phase quadrature. In this manner one could employ sharp filters and then one would operate on the filtered signals to demodulate them. In the present state of the art the techniques use in phase and quadrature and are referred to as I & Q systems. As indicated, these systems operate at baseband and have the capacity of detecting double sideband/single sideband, AM signals, as well as FSK type signals.
The present state of the art in I & Q channel receivers uses passive components for processing the baseband signals so that they can be demodulated. In the use of passive components one cannot employ feedback means to stabilize the receivers. In such systems each baseband signal may be differentiated, multiplied by the other undifferentiated signal and then summed. The other channel which may be the I or Q channel is treated in a similar manner. This technique is subject to the generation of noise and distortion because of circuit imbalances and imperfections. The phase errors in producing the in phase and quadrature channels generate either noise or distortions. If there is an error in tuning, beat notes are generated which further complicate and cause improper operations.
In addition to the above described problems are unbalanced components such as multipliers and other circuit components which also adds to the noise and distortions. Furthermore, the output of such a system has amplitude variations, requiring tight automatic gain control or additional complicated circuitry to circumvent the resulting imperfections.
One can use digitizing techniques to operate on the baseband signals and then to demodulate them according to such techniques. In any event, this does not eliminate the above described problems and will further introduce additional problems of aliasing and high level signal generation as necessary to operate with digital equipment.
It is therefore an object of the present invention to provide a simple demodulator which employs two channel operation while avoiding the above noted problems. It is a further object to provide an improved baseband demodulator which employs automatic frequency control (AFC) to thus eliminate the off tuned condition.
As will be described, the demodulator according to this invention eliminates the deliterious effect of small imperfections in phase splitting the local oscillator. In addition, the circuit to be described can provide an automatic gain control (AGC) signal to allow higher dynamic range operation. In providing such features the circuit does not require high gain baseband signal amplifiers. In this manner the circuit can use fixed gain amplifiers or low gain amplifiers using AGC. This has the effect of permitting the use of amplifiers with large feedback factors so that the gain and balance between the two channels is improved.
As will be described, the demodulator according to this invention is analog in nature but is of such a configuration which enables it to be simply integrated on a circuit chip.